What’s behind Trump’s bounce?

I’ve been waiting to come across a post like this from a smart polling guy about Trump’s bounce and Mark Blumenthal has come through. Yes, this is an analysis based on a single polling organization, but what it has to say is very telling:

The recently narrowed race coincides with a shift in party identification in SurveyMonkey’s tracking, particularly over the past two weeks. While Democrats had held a remarkably consistent party ID advantage since January (usually in the 4–8 point range), that margin narrowed to three points last week (33 to 29 percent) and just a single percentage point (32 to 31 percent) this week. [bold mine, italics original]

The shift is also evident when including the partisan leanings of those who are initially independent. For most weeks in 2016, SurveyMonkey’s tracking gave Democrats an advantage of 3 to 5 percentage points in leaned party identification. Last week, the same number of voters (43 percent) identified or leaned with the Republicans as identified with or leaned to the Democrats. This week, Republicans have a one percentage point edge (43 to 42 percent).

While aggregate partisanship and vote preference has shifted in our samples over the past four weeks, vote preference by party has remained mostly stable. Trump may have gained very slightly among Republicans and independents who lean Republican (rising from 86 to 90 percent), although his vote has fallen (from 37 to 32 percent) among independents who lean to neither party.

Blumenthal has a lot of smart thoughts on all this, but I think it is pretty simple. What’s more likely, 1) that the Republican convention has led to a substantive change in mass partisanship throughout the country? or 2) that the Republican convention has changed the partisan balance of Americans responding to survey questions?

Here’s the key chart:

I will confidently predict that if Hillary Clinton and Donald Trump both win 90-91% of their partisans, Clinton wins the election by a close, but comfortable margin.